docs: def plot_cylinder(ax, start, end, start_radius, end_radius,
                    color='black', alpha=1., linspace_count=_LINSPACE_COUNT):
      '''plot a 3d cylinder'''
      assert not np.all(start == end), 'Cylinder must have length'
      x, y, z = generate_cylindrical_points(start, end, start_radius, end_radius,
                                            linspace_count=linspace_count)
      ax.plot_surface(x, y, z, color=color, alpha=alpha)